What is a Length Unit Converter?
The length unit converter switches between mm, cm, m, km, inches, feet, yards, and miles.
How it's calculated
1 inch = 25.4 mm (exact), 1 foot = 304.8 mm, 1 yard = 0.9144 m, 1 mile = 1,609.344 m
Units that are easy to mix up
| Units | Why it's confusing | How to tell them apart |
|---|---|---|
| mile vs nautical mile | Both called "mile" | 1 mile=1,609m (Roman pace-based), 1 nautical mile=1,852m (1 minute of latitude) — different origins entirely |
| inch vs foot | Both body-derived | 1 inch=2.54cm, 1 foot=12 inches=30.48cm |
Inches and feet came from the human body
Foot was literally the length of a foot; yard is said to derive from the distance from nose to fingertip (or possibly King Henry I's arm length); inch came from the width of a thumb. Body-based units meant you never needed to carry a ruler, but they varied from person to person, which is exactly why the meter — originally defined as a fraction of Earth's meridian — was created.
It wasn't until 1959 that 1 inch was internationally standardized to exactly 25.4mm — before that, US and British inches differed by a tiny amount.
Good to know
- Inches show up everywhere in daily life — TV and monitor diagonals, bicycle wheels, jeans waist sizes, and pipe fittings are all still measured in inches even in metric countries.
- The oddly specific figure of 1 mile = 1,609m comes from ancient Rome: mille passus, meaning 1,000 paces of a marching soldier.
- The nautical mile (1,852m) is different — it's defined as one minute of latitude, which made navigation calculations straightforward on nautical charts.
- The definition of a meter has changed three times — from Earth's meridian, to a physical platinum bar, to today's definition based on the distance light travels in 1/299,792,458 of a second.
- The 1999 loss of NASA's Mars Climate Orbiter was traced to a unit mismatch — one team used pound-seconds, another used newton-seconds, and the $300 million spacecraft was lost.
Frequently asked questions
Q. How many cm is a 32-inch TV?
About 81cm diagonal. Multiply inches by 2.54.
Q. How do I quickly estimate feet to meters?
Divide by 3 for a rough estimate. 6 feet is about 2m (precisely 1.83m).
Q. Why are miles and nautical miles different?
Miles derive from Roman military pacing, while nautical miles derive from a minute of Earth's latitude — completely different origins.